1. Field of the Invention
The present invention relates to a process for preparing alumina trihydrate having significantly greater whiteness and brightness than commonly available alumina trihydrate. More particularly, the present invention relates to a process for increasing the G. E. brightness and ASTM whiteness level of alumina trihydrate to levels of at least about 98% and 99%, respectively, and with a thin, platelet structure suitable for use as a paper filler or coating material for whitening and brightening purposes in a papermaking process.
2. Description of the Related Art
Methods for producing white alumina trihydrates from sodium aluminate solutions are known, as is the application of the hydrates in papermaking processes, for use in both coatings and fillers to improve whiteness and brightness. For example, in U.S. Pat. No. 3,832,442, which issued on Aug. 27, 1974, to Robert B. Emerson, there is disclosed a process for producing alumina trihydrates of assertedly higher purity and greater whiteness. The disclosed process involves contacting with activated alumina a solution of sodium aluminate in slurry form that can be provided as a by-product of the Bayer process. The solution is sometimes referred to as a "Bayer process liquor," and the alumina serves as a purifying agent to remove impurities from the liquor so that alumina trihydrates that are precipitated from the purified solution have improved whiteness as compared with precipitates from untreated liquor, from carbonaceous filter treated liquor, and from alpha-methyl cellulose treated liquor. However, although the product resulting from the disclosed process had improved whiteness over pre-existing approaches, even higher whiteness levels are desirable.
A patent that discloses a process for preparing an alumina monohydrate for use as a pigment or filler in papermaking processes is U.S. Pat. No. 4,946,666, which issued on Aug. 7, 1990, to Neil Brown. The product of that process is a flat, crystalline boehmite, the crystals having an hexagonal shape and a particle size of between 0.2 and 0.8 microns. The process includes an oxidation step that requires autoclaving of the Bayer liquor under vigorous agitation to induce nucleation of crystalline boehmite upon cooling.
Although the known processes provide alumina having a satisfactory whiteness level, it is an object of the present invention to provide a process for preparing an alumina trihydrate that has even greater brightness and whiteness and that also is suitable for use in papermaking processes as a brightness and whitening agent for providing paper of improved brightness and whiteness.